Dual, or back-to-back, proportioning valves have been used in hydraulic braking systems for vehicle brakes, particularly on cross-split automotive braking systems. A cross-split braking system includes a first braking system defined by one master cylinder chamber interconnected with one front and an opposite rear brake hydraulic system. A second braking system includes the second master cylinder chamber interconnected with the other front and an opposite rear hydraulic braking system.
Both separate and back-to-back proportioning valves have been used in the fluid path between the master cylinder chamber and the rear brake hydraulic actuator, such as the rear brake wheel cylinder, in cross-split braking systems. Such proportioning valves permit the brake pressure at the rear wheels to rise in direct portion to the input pressure received from the master cylinder chamber until a predetermined pressure is obtained. Thereafter, the brake pressure at the rear wheels rises at a lesser rate than the input pressure from the master cylinder chamber. In such proportioning valves, should brake failure occur in one of the first and second braking systems brake pressure continues to be applied to the other half of the system. Brake pressure to the rear wheel brake of the operative system continues to be transmitted through the proportioning valve associated with the rear wheel brake. In such a failure condition, no means are provided for bypassing the proportioning valve to permit full brake pressure to be applied to the rear wheel brake.
As shown in U.S. Pat. No. 3,480,333, proportioning valves have been used in split braking systems, wherein one master cylinder chamber is interconnected with both front brakes, and the other master cylinder chamber is interconnected with both rear brakes. The front and rear brakes comprise separate braking systems. Typically, the front braking system includes a metering valve arrangement. A proportioning valve is provided in the fluid path between the master cylinder and both of the rear brakes. If the front brakes fail, a bypass valve is provided to bypass the proportioning valve of the rear brakes in order to apply full pressure on both rear brakes.